In the field of wire matrix printers, it has been quite common in the past to provide a print head which has included a plurality of print wire actuators or solenoids arranged or grouped in a manner to drive the print wires a precise distance from a rest or home position to an impact position. The print wires are generally secured to the solenoid plunger which is caused to be moved the precise distance when the solenoid coil is energized, the plunger operating against the action of a return spring.
Representative prior art is shown in U.S. Pat. No. 3,755,700 issued to H. Buschmann et al. on Aug. 28, 1973, which discloses an electromagnetic drive having a pair of pole shoes and an armature guided in the pole shoes wherein the armature consists of two segments, one being a non-magnetic segment which receives one end of a helical spring and the other end of the spring engaging a pin. The pin is provided with a projection which corresponds to the inner diameter of the spring so that the cylindrical form of the spring is maintained during operation by the chamber of the above-mentioned armature segment. This segment also serves as a guiding element which insures that the armature moves freely in the longitudinal direction of the magnet coil.
U.S. Pat. No. 3,787,791 issued to J. H. Borger et al. on Jan. 22, 1974, shows a solenoid having a plunger which is guided along the coil bobbin and which also includes a guide piece for the print wire which is secured to the armature plunger by a swaging operation.
U.S. Pat. No. 3,802,543 issued to R. Howard on Apr. 9, 1974, shows a solenoid which includes a shell housing a coil with a central core portion for receiving a print wire which is secured to one end of an armature and which print wire passes through an opening in the shell. The armature is slidable in a bobbin portion of the coil which portion abuts one end of a central core portion with an air gap between the armature and the central portion.
U.S. Pat. No. 3,897,865 issued to D. P. Darwin et al. on Aug. 5, 1975, discloses a solenoid for driving a print wire through an aperture in a fixed pole piece and which wire is attached to a movable armature. The armature and the pole piece are disposed in the aperture of the coil which is wound about a bobbin and wherein the hollow inner portion forms a core or working gap of the solenoid drive system. A compression return spring is mounted in a recess or counter bore of the fixed pole piece and the spring is under compression with the pole piece held in place under a force or friction fit in the armature. The clearance of the armature allows it to slide freely in the inner core of the bobbin and the armature is held in place by a stop member.
U.S. Pat. No. 3,900,094 issued to R. B. Larson et al. on Aug. 19, 1975, shows a solenoid actuator assembly which has an armature secured to the print wire and disposed for slidable axial reciprocation within a cylindrical bearing between a front stop and a rear stop. The coils are each insulated by a plastic bobbin. The front stop is of hollow configuration to provide a central bore and a reduced end portion which end portion is provided with a flared extremity surrounding the end of the bore.
U.S. Pat. No. 4,004,671 issued to N. Kondur, Jr. on Jan. 25, 1977, discloses a solenoid with a drive coil wound on a bobbin or spool and a front pole piece is inserted into one end of a tube and has an enlarged flange which is interposed between a nose portion and the spool and a central bore in the front pole piece is aligned with a central opening in the nose portion. The second pole piece is interposed between the spool and a stop which has a sleeve sized to receive a return spring in surrounding relation to the armature.
And, U.S. Pat. No. 4,016,965 issued to R. L. Wirth et al. on Apr. 12, 1977, shows a solenoid which includes a stationary pole piece and a movable plunger with one end of the plunger being flanged so that the surface of the housing functions as a pole piece for one end of the plunger. The stationary pole piece is pressed into the end of the housing after the bobbin with coil windings is placed inside the housing. The end of the plunger performs two functions of providing a seat for the return spring and also a flux path from the air gap to the plunger.